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Abstract

More than 30 Curcuma species (Zingiberaceae) are found in Asia, where the rhizomes of these plants are
used as both food and medicine, such as in traditional Chinese medicine. The plants
are usually aromatic and carminative, and are used to treat indigestion, hepatitis,
jaundice, diabetes, atherosclerosis and bacterial infections. Among the Curcuma species, C. longa, C. aromatica and C. xanthorrhiza are popular. The main constituents of Curcuma species are curcuminoids and bisabolane-type sesquiterpenes. Curcumin is the most
important constituent among natural curcuminoids found in these plants. Published
research has described the biological effects and chemistry of curcumin. Curcumin
derivatives have been evaluated for bioactivity and structure-activity relationships
(SAR). In this article, we review the literature between 1976 and mid-2008 on the
anti-inflammatory, anti-oxidant, anti-HIV, chemopreventive and anti-prostate cancer
effects of curcuminoids. Recent studies on curcuminoids, particularly on curcumin,
have discovered not only much on the therapeutic activities, but also on mechanisms
of molecular biological action and major genomic effects.

Background

Curcuma species

In Asia zingiberaceous plants have been used since ancient times as both spices and
medicines, such as in traditional Chinese medicine. Within this plant family, various
Curcuma species, particularly C. longa (turmeric), C. aromatica (wild turmeric), and C. xanthorrhiza (Javanese turmeric), have been used. The rhizomes of these plants are usually aromatic
and carminative, and are used to treat indigestion, hepatitis, jaundice, diabetes,
atherosclerosis and bacterial infections [1,2].

The rhizomes of C. longa, commonly known as turmeric, are used worldwide as spices (e.g. curry), flavoring
agents, food preservatives and coloring agents. They are also used as medicines to
treat inflammation and sprains in India, China and other Asian countries. Curcuminoids,
the main components in Curcuma species, share a common unsaturated alkyl-linked biphenyl structural feature and are
responsible for their major pharmacological effects. The biological and chemical properties
of curcuminoids were reported [4-9].

Curcuminoids in C. longa and other Curcuma species are mainly curcumin (2), bis-demethoxycurcumin (3) and demethoxycurcumin (4) (Figure 1), among which curcumin is the most studied and shows a broad range of biological
activities. This article highlights some of the important biological properties of
curcumin and its derivatives, as well as their structure-activity relationships (SAR).

C. xanthorrhiza is used as a tonic in Indonesia and a choleric drug in Europe. Apart from curcuminoids,
this species contains bioactive bisabolane-type compounds, such as α-curcumen (5), ar-turmerone (6) and xanthorrhizol (7) (Figure 2). These three compounds demonstrated strong anti-cancer activities against Sarcoma
180 ascites in mice [10-15]. In addition, xanthorrhizol (7) exhibited antibacterial activity [16].

Curcumin and its biological activities

Curcumin (2) [diferuloylmethane, 1,7-bis-(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione]
is the main yellow constituent isolated from C. longa and other Curcuma species. It was first isolated in 1870, but its chemical structure had not been elucidated
until 1910 [17] and was subsequently confirmed by synthesis. Curcumin has a unique conjugated structure
including two methylated phenols linked by the enol form of a heptadiene-3,5-diketone
that gives the compound a bright yellow color.

In addition to its well known anti-inflammatory effects, curcumin also possesses other
therapeutic effects on numerous biological targets [18]. Other activities of curcumin include reduction of blood cholesterol level, prevention
of low density lipoprotein (LDL) oxidation, inhibition of platelet aggregation, suppression
of thrombosis and myocardial infarction, suppression of symptoms associated with type
II diabetes, rheumatoid arthritis, multiple sclerosis and Alzheimer's disease, inhibition
of human immunodeficiency virus (HIV) replication, enhancement of wound healing, increase
of bile secretion, protection from liver injury, cataract formation and pulmonary
toxicity and fibrosis, exhibition of anti-leishmaniasis and anti-atherosclerotic properties,
as well as prevention and treatment of cancer [18]. Curcumin is non-toxic even at high dosages, and has been classified as 'generally
recognized as safe' (GRAS) by the National Cancer Institute [19]. There were also studies focusing on the biology and action mechanisms of curcumin
[18,20].

Synthetic bioactive curcumin analogs were developed from the natural compound based
on the structure-activity relationship (SAR) studies and optimization of compounds
as drug candidates in their relations to different activities, including anti-inflammatory,
anti-oxidant, anti-HIV, chemopreventive and anti-cancer (prostate cancer), as well
as possible action mechanisms.

Pro-inflammatory cytokines, such as interleukin-1β (IL-1β) and tumor necrosis factor-α
(TNF-α), play key roles in the pathogenesis of osteoarthritis (OA). Anti-inflammatory
agents that can suppress the production and catabolic actions of these cytokines may
have therapeutic effects on OA and some other osteoarticular disorders. Accordingly,
curcumin was examined for its effects on IL-1β and TNF-α signaling pathways in human
articular chondrocytes in vitro [22]. Expression of collagen type II, integrin β1, cyclo-oxygenase-2 (COX-2) and matrix
metalloproteinase-9 (MMP-9) genes was monitored by Western blotting. The effects of
curcumin on the expression, phosphorylation, and nuclear translocation of protein
components of the NF-κB system were studied with Western blotting and immunofluorescence
respectively. The results indicated that curcumin suppressed IL-1β-induced NF-κB activation
via inhibition of inhibitory protein κBα (IκBα) phosphorylation, IκBα degradation,
p65 phosphorylation and p65 nuclear translocation. Curcumin also inhibited IL-1β-induced
stimulation of up-stream protein kinase B Akt. These events correlated with the down-regulation
of NF-κB targets, including COX-2 and MMP-9. Similar data were obtained when chondrocytes
were stimulated with TNF-α. Curcumin also reversed the IL-1β-induced down-regulation
of collagen type II and β1-integrin receptor expression. These results indicate that
curcumin may be a naturally occurring anti-inflammatory nutritional agent for treating
OA via suppression of NF-κB mediated IL-β/TNF-α catabolic signaling pathways in chondrocytes
[22]. Curcumin was found to act by diverse anti-inflammatory mechanisms at several sites
along the inflammation pathway [23].

Anti-inflammatory SAR

The active constituents of C. longa are curcuminoids, including curcumin (2), demethoxycurcumin (3) and bisdemethoxycurcumin (4) [24] (Figure 1), among which curcumin is the most potent anti-inflammatory agent [25]. In addition to these natural curcuminoids, sodium curcuminate (8) and tetrahydrocurcumin (9) (Figure 3) showed potent anti-inflammatory activity at low doses in carrageenin-induced rat
paw edema and cotton pellet granuloma assays [26]. Other semi-synthetic analogs of curcumin were screened for anti-inflammatory activity
in the same assays; diacetylcurcumin (10) and tetrabromocurcumin (11) (Figure 3) were the most potent [27,28]. The presence of the β-diketone moiety as a linker between the two phenyl groups
was deemed important for the anti-inflammatory activity.

Nurfina et al. designed and synthesized 13 symmetrical curcumin analogs (12–24) [29]. Anti-inflammatory activity was evaluated by inhibition of carrageenin-induced swelling
of rat paw (Table 1); and the following SAR conclusions were drawn: (a) appropriate substituents on the
phenyl rings were found necessary for anti-inflammatory activity. Unsubstituted compound
12, ortho-methoxy, substituted analog 18, and meta-methoxy substituted analog 13 showed no inhibitory activity; (b) proper substituents at the para-positions of the phenyl rings were also crucial. A para-phenolic group leads to the most potent anti-inflammatory activity [compare 3 (p-OH), 21 (p-CH3), 20 (p-OCH3), 19 (p-Cl) as well as 2 with 22 and 24 with 14]; and (c) size of the substituents adjacent to a para-phenol was found to be important for potency. Dimethyl substitution (15) at R2 and R4 enhanced the activity most, followed by diethyl (16) and dimethoxy (24). Compound 21 with two isopropyl moieties showed weaker activity, while 23 with bulky tetrabutyl substitution at both positions showed no anti-inflammatory activity.

Cyclovalone (25) and three analogs (26–28) (Figure 4) having a cyclohexanone or cyclopentanone in the linker between the two phenyl rings
showed anti-inflammatory activity to inhibit cyclooxygenase [30]. Compounds 26–28 were more potent than curcumin (2) which was used as a reference standard. The dimethylated 28 and 26 were more potent than 27 and 25 respectively, and thus, the addition of methyl groups on the phenyl rings enhanced
anti-inflammatory activity. The increased size of the cycloalkanone ring, by replacing
the cyclopentanone in 27 with a cyclohexanone in 25, increased inhibitory potency. However, this effect was not seen in the dimethylated
compounds 28 and 26 respectively, both of which were comparably potent.

Besides curcumin, other structurally related constituents of plants in the Zingiberaceae
family possess anti-inflammatory activity [31]. Examples are the phenolic yakuchinones A and B (29 and 30) isolated from Alpinia oxyphylla [32-34] (Figure 5).

Anti-oxidation

Anti-oxidant activity

Most natural anti-oxidants can be classified into two types of compounds, namely phenolic
and β-diketone [35]. Sesaminol isolated from sesame belongs to the former, while n-triacontane-16,18-dione isolated from the leaf wax of Eucalyptus belongs to the latter. Curcumin (2) is one of the few anti-oxidants that possess both phenolic hydroxy and β-diketone
groups in one molecule. Its unique conjugated structure includes two phenols and an
enol form of a β-diketone. Therefore, it may have a typical radical trapping ability
and a chain-breaking anti-oxidant activity.

Curcumin is a potent anti-oxidant whose action mechanism is not well understood. However,
the nonenzymatic anti-oxidant process of a phenolic compound is generally thought
to have two stages as follows:

S-OO• + AH ↔ SOOH + A•

A• + X• → nonradical materials

Where S is the oxidized substance; AH is the phenolic anti-oxidant; A• is the anti-oxidant
radical; and X• is another radical species or the same species as A• [35]. While the first stage is reversible, the second stage is irreversible and must produce
stable radical terminated compounds. Structural elucidation of the terminated compounds
may contribute significantly to understanding the mechanism of the phenolic anti-oxidant.
It has recently been shown that dimerization is a main termination process of the
radical reaction of curcumin itself. In food, the anti-oxidant coexists with large
amounts of oxidizable biomolecules, such as polyunsaturated lipids. These biomolecules
were found to produce reactive peroxy radicals during their oxidation, which may act
as X• and couple with the anti-oxidant radical (A•) in the second step of the above
anti-oxidation scheme [36].

Anti-oxidant SAR

Curcumin showed both anti-oxidant and pro-oxidant effects in oxygen radical reactions.
Depending on the experimental conditions, it may act as a scavenger of hydroxy radicals
or a catalyst in the formation of hydroxy radicals [37-39]. The anti-oxidant effect of curcumin presumably arises from scavenging of biological
free radicals.

The anti-oxidant activities of three natural curcuminoids (2–4) and their hydrogenated analogs (9, 31, 32) (Figure 6) were examined in three bioassay models, i.e. the linoleic acid auto-oxidation model,
rabbit erythrocyte membrane ghost system, and rat liver microsome system. The results
obtained from the three models were consistent. Curcumin (2) and tetrahydrocurcumin (9) had the strongest anti-oxidant activity among the natural and hydrogenated curcuminoids
respectively [35]. Among all six compounds, tetrahydrocurcumin (9) showed the highest potency, implying that hydrogenation of curcuminoids increased
their anti-oxidant ability. Absence of one or both methoxy groups resulted in decreased
anti-oxidant activity in both natural curcuminoids and tetrahydrocurcuminoids. In
contrast, Sharma et al. reported that the presence of methoxy groups in the phenyl rings of curcumin enhanced
anti-oxidant activity [40].

Venkatessan et al. [41] used three models to investigate the importance of the phenolic hydroxy groups, as
well as other substituents on the phenyl rings of curcuminoids, to anti-oxidant activity.
The three anti-oxidant bioassays were inhibition of lipid peroxidation, free radical
scavenging activity by the DPPH method, and free radical scavenging activity by the
ABTS method. The data and compound structures are shown in Table 2. Generally, curcumin analogs with a phenolic moiety were more potent than non-phenolic
analogs, and thus, phenolic substitution is important for anti-oxidant activity. Compound
15, a 4'-hydroxy-3',5'-dimethyl substituted analog, showed potency in all three bioassays.
However, compound 23, a 4'-hydroxy-3',5'-di-t-butyl analog, was ten-fold less potent in the lipid peroxidation assay, indicating
that steric hindrance at the positions flanking the hydroxyl group decreased anti-oxidative
activity. Changing the 3'-methoxy group in curcumin (2) to an ethoxy group in 33 had little effect on anti-oxidant activity, but both compounds were more potent than
3, which does not have an alkoxy group at the 3'-position. In all three systems, tetrahydrocurcumin
(9) and curcumin (2) showed comparable activity. This result suggests that enhanced electron delocalization
of the double bonds may not be essential to anti-oxidant activity of curcuminoids.

The anti-oxidant mechanisms of curcumin have been investigated. The salient finding
is that curcumin is a phenolic chain-breaking anti-oxidant, which donates H atoms
from the phenolic groups [42-47]. However, some contrasting results suggest that H atom donation takes place at the
active methylene group in the diketone moiety [48,49]. Ligeret et al. evaluated the effects of curcumin and numerous derivatives on the mitochondrial
permeability transition pore (PTP), which can release apoptogenic factors from mitochondria
to induce apoptosis [50]. The authors postulated that PTP opening is closely related to the anti-oxidant property
of curcumin. Based on the data on mitochondria swelling, O2• and HO• production, thiol oxidation and DPPH• reduction, the authors concluded that
phenolic groups in curcuminoids are essential for activity, and are more effective
at the para position than at the ortho position. In addition, an electron donating group at the ortho position relative to the phenolic group is also required for activity, while t-butyl and bulky substituents are not favorable. In contrast, electron-withdrawing
substitution, such as NO2, reduced activity. Although ferulic acid does not show anti-oxidant effects, replacing
the β-diketone moiety of curcumin with a cyclohexanone ring attenuated anti-oxidant
activity. Thus, the authors concluded that the β-diketone contributed to, but could
not induce, the activity of curcumin derivatives. The conclusions agree with the prevailing
SAR for anti-oxidant activity.

However, in one study, a curcumin analog without phenolic and methoxy groups was found
to be as potent as curcumin in terms of scavenging hydroxy radicals and other redox
properties [51]. Wright employed theoretical chemistry to interpret the controversy [52]; taking into account the diversity of test free radicals, solvents, and pH ranges
used in the literature. First, he explored the stabilities of curcumin conformers,
pointing out that the enol form is the most stable, followed by the trans-diketo form, and then the cis-diketo form (Figure 7). Calculations showed that the phenolic O-H is the weakest bond in curcuminoids.
This theoretical approach favors the necessity of a phenolic OH group for the anti-oxidant
activity of curcumin and its analogs. However, the C-H bond of the methylene group
becomes active when radicals with high bond dissociation enthalpy, such as methyl
and t-butoxy radicals, are used. Thus, differences among experimental results can be possibly
due to the differences in the attacking radicals used in different bioassay systems.

Anti-HIV

Anti-HIV activity

Oxidative stress is implicated in HIV-infection. It was suggested that plant anti-oxidants
may offer protection from viral replication and cell death associated with oxidative
stress in patients with HIV/acquired immune deficiency syndrome (AIDS) [53]. Curcumin (2) can inhibit purified HIV type 1 integrase, HIV-1 and HIV-2 protease, and HIV-1 long
terminal repeat-directed gene expression of acutely or chronically infected HIV-1
cells. Curcumin can also inhibit lipopolysaccharide-induced activation of NF-κB, a
factor involved in the activation and replication of HIV-1. However, curcumin did
not show significant efficacy in clinical trials.

Anti-HIV SAR

In addition to reverse transcriptase and protease, HIV-1 integrase is being explored
as a new target for the discovery of effective AIDS treatments. HIV-1 integrase is
the enzyme that catalyzes the integration of the double-strained DNA of HIV into the
host chromosome [54]. Curcumin inhibited this activity of HIV-1 integrase [54]. Other classes of compounds inhibited HIV-1 integrase in enzyme assays, but few showed
specificity against HIV-1 integrase and even fewer were active in cell-based assays
[55]. Curcumin was reported to have moderate activity in cell-based assays, in addition
to its activity in enzyme assays [56].

Therefore, modified curcumin analogs were developed for anti-HIV potency as well as
action mechanism studies [54,57]. Mazumder et al. [57] synthesized curcumin analogs (Table 3) as probes to study the mechanism of anti-HIV-1 integrase. Evidence suggests that
curcumin does not bind to HIV-1 integrase at either the DNA-binding domain [58] or the binding site of another HIV-1 integrase inhibitor, i.e. NSC 158393 [59]. Compounds without a hydroxy group on the phenyl ring (12, 20) did not inhibit HIV-1 integrase. Therefore, hydroxy groups on the phenyl rings are
apparently essential for inhibitory activity. Compounds 35 and 36, which contain two and one catechol ring respectively, exhibited much greater activity
than curcumin (2), indicating that replacing one or both methoxy groups on curcumin with hydroxy groups
increased anti-HIV activity. Tetrahydrocurcumin (9), with a saturated linker between the phenyl groups, did not show inhibitory activity
in this assay, suggesting that an unsaturated linking group also contributed to activity.
In addition, compound 37, with a unique linker bridging two catechol rings, showed potency comparable to that
of 35 and 36, and greater than that of 2.

In the further SAR investigation of curcumin analogs as inhibitors of HIV-1 integrase,
a syn disposition of the C=C=C=O moiety in the linker and a coplanar structure were found
to be important to the integrase inhibitory activity of curcumin analogs [55]. The experimental results are consistent with the quantitative structure-activity
relationships (QSAR) computed with MOE (Chemical Computing Group, Canada) and Cerius2
(Molecular Simulations, USA) programs [60]. Figure 8 summarizes the anti-HIV-1 integrase SAR of curcumin analogs. However, no therapeutic
indices were reported for the tested compounds.

Chemoprevention

Chemoprevention is a relatively new concept. It attempts to intervene at early stages
of cancer before the invasive stage begins [61]. Nontoxic agents are administered to otherwise healthy individuals who may be at
increased risk for cancer. Some potential diet-derived chemopreventive agents include
epigallocatechin gallate in green tea, curcumin in curry and genistein in soya. Curcumin
demonstrated a wide-range of chemopreventive activities in preclinical carcinogenic
models of colon, duodenum, fore-stomach, mammary, oral and sebaceous/skin cancers.
The National Cancer Institute is conducting Phase I clinical trials of curcumin as
a chemopreventive agent for colon cancer [62]. Curcumin's chemopreventive mechanisms are pleiotropic. It enhanced the activities
of Phase 2 detoxification enzymes of xenobotic metabolism, including glutathione transferase
[63] and NADPH:quinone reductase [64]. It also inhibited pro-carcinogen activating Phase 1 enzymes such as cytochrome P450
1A1 [65]. As regards its mode of chemopreventive action in colon cancer, curcumin exhibited
diverse metabolic, cellular and molecular activities including inhibition of arachidonic
acid formation and its further metabolism to eicosanoids [66].

Anti-prostate cancer

Prostate cancer is the most common cancer among males in the West [67] and is a complex heterogeneous disease that affects different men differently. The
cause of prostate cancer is largely unknown. However, androgen and the androgen receptor
(AR) are postulated to play crucial roles in the development of prostate cancer [68].

Prostate cancer is currently treated with a combination of surgery, radiation and
chemotherapy. The therapeutic agents used clinically include steroidal anti-androgens,
such as cyproterone acetate, and non-steroidal anti-androgens, such as flutamide and
bicartamide. The steroidal anti-androgens possess partial agonistic activity and overlapping
effects with other hormonal systems, leading to complications such as severe cardiovascular
problems, gynecomastia, libido loss and erectile dysfunction [69-71]. Non-steroidal anti-androgens have fewer side effects and higher oral bioavailability
than steroidal anti-androgens.

While non-steroidal anti-androgens are advantageous, anti-androgen withdrawal syndrome
was found in patients receiving non-steroidal anti-androgens for several months [72,73]. Long-term drug usage would lead to mutation of the AR, and the non-steroidal anti-androgens
may exhibit agonistic activity to the mutant AR [74]. In addition, the clinically available anti-androgens are unable to kill prostate
cancer cells, and within one to three years of drug administration, the cancer usually
develops into an androgen refractory stage [72-74]. Therefore, new classes of anti-prostate cancer drugs are urgently needed.

Prostate cancer occurs much less frequently in Asia than in the West [75], possibly due to dietary differences. Turmeric is much more highly consumed as both
spice and medicine in India, Thailand, China and Japan than in the West. Thus, we
and other researchers investigated turmeric and its constituent curcumin for anti-prostate
cancer effects.

Although curcumin is a well known anti-inflammatory and anti-oxidant agent, its anti-prostate
cancer activity has not been extensively explored. Over the last decade, our research
group has used curcumin (2) as a lead compound for the design and synthesis of curcumin analogs as a new class
of potential anti-androgenic agents for the treatment of prostate cancer as well as
for action mechanism studies [76-81]. Certain curcumin analogs including 38 (JC-9), 39 (4-ethoxycarbonyl curcumin, ECECu) and 40 (LL-80) (Figure 9), showed potent in vitro cytotoxic activity against LNCaP and PC-3 human prostate cancer cell lines (Table
4). Among them, compound 40 showed the most potent activity, suggesting that introducing a conjugated side chain
in the enol-ketone linker may stabilize the enol-ketone form as the predominant tautomer
(Figure 9), which may contribute to the anti-prostate cancer activity. Although the entire
structure of the AR has not been fully determined and the mechanism of how curcumin
derivatives interact with the AR is still unclear, preliminary studies showed that
these curcumin derivatives inhibit AR function via an AR degradation pathway, which plays an important role in the growth of prostate
cancer [82,83]. In addition, compound 38 (JC-9) with its potent anti-androgenic activity and stable physiological properties
was identified as a lead anti-AR compound. Clinical trials against prostate cancer
are being planned.

We prepared four series of new curcumin analogs [81] including monophenyl curcumin analogs, heterocycle-containing curcumin analogs, curcumin
analogs bearing various substituents on the phenyl rings, and curcumin analogs with
various linkers, which are being tested for their anti-prostate cancer activity and
action mechanism. New curcumin analogs from other research groups [84-86] are also being evaluated for cytotoxic activity against two human prostate cancer
cell lines, i.e. LNCaP and PC-3, and inhibitory activity to the AR, with goals to
elucidate more refined SAR and optimize curcumin analogs to develop better anti-prostate
cancer drugs.

Conclusion

Natural curcuminoids are compounds found in Curcuma species, which are used as a medicine of the upper class of traditional Chinese medicine
herbs that are generally not toxic and are in rich content in natural foods and spices.
Curcuminoids and other natural and synthetic curcuminoids possess various bioactivities
including anti-inflammatory, anti-oxidant, anti-HIV, chemopreventive and anti-prostate
cancer effects. In addition, curcumin was recently found to prevent experimental rheumatoid
arthritis [87]. Recent studies on curcuminoids, particularly on curcumin, have discovered not only
much on the therapeutic activities, but also on mechanisms of molecular biological
action and major genomic effects. Our research group developed some anti-androgenic
curcumin analogs as anti-prostate cancer agents.

de Voogt HJ, Smith PH, Pavone-Macaluso M, de Pauw M, Sucin S: Cardiovascular side effects of diethylstilbestrol, cyproterone acetate, methoxyprogesterone
acetate and estraumustine phosphateused for the treatment of advanced prostate cancer.
Results from European Organization for research on treatment of cancer trials 3076
and 30762.